Leaving stock on drive is useful if you're going to go back and finish with a different tool. For instance, if you rough with a 1/2" ball mill, but finish with a 1/8", you want to leave enough stock remaining that the final surface has only been cut by the 1/8" mill.
Normal surfacing toolpaths do tend to get messy when the surface is vertical or close to vertical.
Since the problem you're having is with vertical walls, maybe you should use a Pocket toolpath for that section of the part instead of a surfacing toolpath. It will give better results.
Another option is the Surface Parallel Steep toolpath. I haven't used it, but it's supposed to be optimized for steep surfaces.
By the way, there's a MasterCAM forum here.
Mastercam - CNCzone.com-The Largest Machinist Community on the net!
You'll get more answers from that forum. Not many Tormach owners can afford to pay MasterCAM's fees. Me included.